1. Field of the Invention
The present invention generally relates to an image capturing device and an image delivery method which can be suitably applied to a video conference, and more particularly, to provide an immediate image capturing device and image delivery method without adding the load of the computer.
2. Description of the Prior Art
With the development of technology, the video conference has become a popular tool used by many companies and users to transmit messages to each other via remote terminals. The image with high resolution is thus required. However, limited by the hardware structure and the network bandwidth, the transmission process is easily influenced by the computer process efficiency and the network bandwidth, so the quality of the transmitted image is always poor. Generally, the high definition (HD) image means the image with 1280*720 pixels or more. It thus becomes a heavy load for a computer when a conventional video conference device is used to capture HD frames and show them on the screen of the computer.
Please refer to FIG. 1. FIG. 1 is a functional block diagram illustrating the conventional image process of the camera module 1. The light is gathered to an image sensor 102 through the optical lens 100, and then the image sensor 102 transforms the light signal into an electric signal. The electric signal is a raw pixel data Sr. Afterward, the raw pixel data is successively processed in the image processor 104 and is transformed into a YUV color space pixel data SYUV. The YUV pixel data can be compressed into a compression bitstream Sc through the image compressor 106, the common compression standard is JPEG format. A path data is selected by the multiplexer 108 from the YUV pixel data or the compression bitstream, and the selected path data is transmitted to the USB bridge 120. The USB bridge 120 transforms the video data into the USB signal and transmits the USB signal to the computer 2. The above-mentioned structure is widely applied to a Web-Camera on a personal computer (PC) or a notebook.
However, under the USB 2.0 high-speed standard, the transmitting speed limitation to an isochronous bandwidth is 196.6 Mbps (Mega-bits per second), and the bandwidth can contain the raw data amount of 30 VGA (640*480) frames per second. However, when the frame resolution reaches 30 1280*720 frames per second, the data amount will reach 331.78 Mbps, which is obviously over the maximum bandwidth of USB 2.0. In order to transmit this amount of video signal in the conventional structure, the compression technology is used to reduce the data amount, or decrease the number of frames transmitted per second. However, if the compression technology is used, extra processes are needed and the load of the processor will become heavier. If the method of decreasing the number of frames transmitted per second is used, the user will feel that the frames shown on the screen are unsmooth.
When the conventional camera module uses the JPEG standard to compress an image, a compression rate of 5 to 10 times, namely a data amount of 66.37 Mbps to 33.18 Mbps, can be provided to be transmitted to the computer through the USB. However, if the data is transmitted via the network, the application will be limited by the bandwidth of the network. The network bandwidth can be sorted into two classes. The first class is a local-area network, and the second class is a wide-area network. The local-area network can provide an upload/download transmitting speed of 50 Mbps (100 Mbps/1 Gbps, for wire network) and 18 to 22 Mbps (for wireless network). The wide-area network can provide a transmitting speed in 30 to 50 Kbps (for modem), 300 Kbps to 1 Mbps (for asymmetric digital subscriber line, ADSL), and 2 Mbps (for fiber to the home, FTTM). Therefore, the JPEG compressing method can be used only in the local-area network, and its application range is relatively limited.
In order to make the data amount smaller and conformed to various bandwidth limitations, the advanced compression technology must be used. Presently, H.264 standard is a well-known compression technology, which is better than MPEG-2 and MJPEG. Using the H.264 technology to compress a video signal with the high resolution of 30 fps, the data transmitting speed can be reduced to 512 Kbps˜2 Mbps (the quality of VHS standard to DVD standard). Thus, it can conform to the bandwidth of the wide-area network in the lower video quality or the static scenes of video conference.
Please refer to FIG. 2. FIG. 2 is a functional block diagram of the software process in the computer 2 under the structure of the conventional camera module 1. First, the USB driving interface 260 receives the USB packets containing JPEG bitstream from the camera module 1 (a JPEG bitstream standard under the conventional structure). Then, the JPEG decoder 262 decompresses the JPEG bitestream data to the original video data in YUV format. Next, the YUV render 266 shows the YUV data on the screen of the computer 2. After that, the H.264 encoder 264 compresses the YUV original data to a video bitstream transmitted to the network by the network transmitter 268. Afterward, the network receiver 270 receives the video bitstream via the network from a remote terminal. Finally, the video bitstream is decompressed to the original video data in YUV form by the H.264 decoder 272, and the YUV data will be shown on the screen of the computer 2 by the YUV render 274.
Under this structure, if the video conference system transmits HD frames at 30 fps, the percentages of CPU loading are shown in table 1. As shown in table 1, when the conventional capturing device for the video conference is used in a notebook with a low level CPU (e.g., Inter Core Due T2200), the total load using rate of the CPU is about 195%. If the conventional capturing device for the video conference is used in a notebook with a high level CPU (e.g., Intel Core 2 Due T7200), the total load using rate of the CPU is still over 155%.
As shown in Table 1, no matter the image is processed by the low level CPU or the high level CPU, the CPU will be seriously over-loaded. Therefore, both of the low level CPU or the high level CPU can not smoothly process the video frame data with HD frames of 30 fps captured by the conventional camera module captures.
TABLE 1CPUIntel CoreIntel Core 2TasksDue T2200Due T7200JPEG decoder 26225%10%H.264 encoder 264>100%>100%YUV render (266)5%5%H.264 decoder 27260%35%YUV render (274)5%5%USB driving interface 260,<10%<10%Network transmitter 268,Network receiver 270Summary>195%>155%
Thereby, the main scope of the present invention is to provide an image capturing device and an image delivery method to solve the problems mentioned above.